aarch64: Fix ICE in aarch64_add_offset_1 [PR94121]

aarch64: Fix ICE in aarch64_add_offset_1 [PR94121]

abs_hwi asserts that the argument is not HOST_WIDE_INT_MIN and as the
(invalid) testcase shows, the function can be called with such an offset.
The following patch is IMHO minimal fix, absu_hwi unlike abs_hwi allows even
that value and will return (unsigned HOST_WIDE_INT) HOST_WIDE_INT_MIN
in that case.  The function then uses moffset in two spots which wouldn't
care if the value is (unsigned HOST_WIDE_INT) HOST_WIDE_INT_MIN or
HOST_WIDE_INT_MIN and wouldn't accept it (!moffset and
aarch64_uimm12_shift (moffset)), then in one spot where the signedness of
moffset does matter and using unsigned is the right thing -
moffset < 0x1000000 - and finally has code which will handle even this
value right; the assembler doesn't really care for DImode immediates if
        mov     x1, -9223372036854775808
or
        mov     x1, 9223372036854775808
is used and similarly it doesn't matter if we add or sub it in DImode.

2020-03-11  Jakub Jelinek  <jakub@redhat.com>

	PR target/94121
	* config/aarch64/aarch64.c (aarch64_add_offset_1): Use absu_hwi
	instead of abs_hwi, change moffset type to unsigned HOST_WIDE_INT.

	* gcc.dg/pr94121.c: New test.